Configurable number of Glance api processes

The Glance API server currently starts a wsgi server on a green thread per request. This appears to limit the processing capability of the Glance API server to a single CPU. Under heavy load this can make Glance appear un-responsive. This is especially noticeable when using SSL, where a single thread can use 100% CPU for encryption and compression processing.

We propose to add multi-process support by starting up a configurable number of processes each listening on the Glance server port.